1. Field of the Invention
The present invention relates to a controller for a microwave oven, and in particular to an improved controller for a microwave oven capable of preventing an erroneous operation thereof causing an undesired heating.
2. Description of the Conventional Art
Generally, it is well known to use a programmable type controller such as a microcomputer so as to control various functions of a microwave oven. However, in the microcomputer, a runaway situation--that is, when the microcomputer is over-loaded by noise--may occur, so that electric power is erroneously supplied to heating elements due to malfunction or the like of the program counter in the microcomputer.
Referring to FIG. 1, a conventional controller for a microwave oven includes an alternating current source 10 for supplying electric power to corresponding elements and power controllers 11 and 12 for controlling the power supply. In addition, a lamp 13 is provided for lighting the interior of a heating chamber (not shown), and a fan motor 14 is provided for driving a fan (not shown) so as to cool a magnetron 19. A transformer 16 is provided to convert electric power into a sufficiently high voltage for driving the magnetron 19 as the electric power is supplied to a primary winding 15 of the transformer 16. A main secondary winding 17, of which its one side is connected to a filament of the magnetron 19 through the capacitor 20 and its other side is connected to a ground, and a sub-secondary winding 18, of which both terminals thereof are connected to the filament of the magnetron 19, supply high voltage of the transformer 16 to the magnetron 19. The magnetron 19 generates a desired energy as the high voltage is supplied thereto from the main- and sub-secondary windings through the above mentioned filament. The capacitor 20 is connected between the main secondary winding 17 and the filament of the magnetron 19 and increases the voltage level of the main secondary winding 17. A diode 21 is connected between the common node of the capacitor 20 and magnetron 19 forming a parallel path with the main secondary winding 17 for preventing the over current voltage in the main secondary winding 17. The electric power controllers 11 and 12 include relay switches 22 and 24, which are driven by relay coils 23 and 25. The relay coil 23 is switched to a DC power supply 26 through a series circuit of a thyristor 29 and a transistor 37, and the relay coil 25 is switched to DC power supply 26 through the transistor 40. In addition, the diodes 27 and 28 are parallely connected with the coils 23 and 25, respectively, for preventing excess current through of the coils 23 and 25. A resistor 31 is connected between the cathode and the gate of the thyristor 29. A resistor 32 and a diode 33 are connected in series between the gate of the thyristor 29 and the start switch 34, in order. A point between the diode 33 and the start switch 34 is connected to an input terminal IN of the microcomputer 30, and the other side of the start switch 34 is connected to a scanning signal output terminal SCN of the microcomputer 30. The base of the transistor 37 is connected to a start signal output terminal S of the microcomputer 30 through the resistor 35, and a resistor 36 connects between the base of the transistor 37 to ground. Here, the thyristor 29 and the transistor 37 form a switching circuit for controlling the supply of electric power supply to the relay coil 23. The base of the transistor 40 is connected to a power control output terminal PWR of the microcomputer 30 through a resistor 38, and a resistor 39 connects the base of the transistor 40 to ground. In addition, there is provided a speaker controller 42 for controlling a speaker 43 in accordance with a signal outputted from an alarm signal output terminal AL of the microcomputer 30, and a display unit 41 for displaying cooking time etc. in accordance with a signal outputted from a display signal output terminal D of the microcomputer 30. The above-described construction corresponds to that disclosed in U.S. Pat. No. 4,481,393 which is incorporated herein by reference.
The operation of the conventional controller for a microwave oven will now be explained with reference to FIGS. 2 and 3.
To begin with, pulse signals 50 through 54 each having a constant duty cycle are outputted from the scanning signal output terminal SCN of the microcomputer 30 as shown in FIG. 2A. The pulse signals 50 through 54 are applied to the gate of the thyristor 29 and the input terminal IN of the microcomputer 30 when the start switch 34 is turned on as shown in FIG. 2B. The microcomputer 30 on receiving the scanning signals detects whether at least two serial pulse signals are inputted thereto so as to avoid an erroneous operation. If the serial pulse signals are detected, the microcomputer 30 checks whether the door of the microwave oven is opened or closed and whether a predetermined heating time is set. As a result, if there is not an abnormal state, the microcomputer 30, as shown in FIG. 2D, outputs a start signal at the time c3 through the start signal output terminal S. In addition, after a predetermined time B is lapsed, at the time c4, the microcomputer 30 checks the signal level inputted to the input terminal IN' at from the point Q. In FIGS. 2A through 2G, since the thyristor 29 is in a cut-off state at the time c4 and thus the signal level at Q becomes a high level voltage, the microcomputer 30 stops the generation of the start signals and stops the heating operation. If the state at the point Q is a high level, the microcomputer does not generate the start signals. In this case, if the relay 23 maintains tire cut-off state continuously, the output signal of the alarm signal terminal AL maintains the cut-off state continuously. However, if the start switch 34, as shown in FIG. 3B, remains turned on for the time c1 through c5, the thyristor 29 becomes conductive by a pulse 53, and the signal level at the point Q becomes low level at the time c4. Therefore, the start signal is generated continuously after the time c4, and the heating operation can be performed.
The power control signal PWR can not be generated before a predetermined time B. If the microcomputer 30 recognizes that the signal level at the point Q is a low level at the time c4, a power control signal is outputted from the power control terminal PWR. However if, at the time c4, the level at the point Q is a high level, the power control signal is not generated, and the relay switch 24 is not closed. At this moment, it means that even if the relay switch 22 is closed because of its malfunction, when the electric potential at the time c4 is a high level, the magnetron 19 is not operational. However, if the microcomputer 30 recognizes that an electric potential at the point Q is a low level, it outputs a pulse signal through the alarm signal terminal AL, and therefore the speaker 43 becomes activated, and the microcomputer 30 displays on the display member 41 that the heating time is counted down. Therefore, when the heating time shown on the display 41 is zero, the microcomputer 30 stops the heating operation.
However, the conventional controller for a microwave oven has disadvantages in that it is directed to turning on a power relay by using only a start switch, so that when a plurality of switches for operating a heating instrument is adopted thereto, each switch disadvantageously requires its corresponding switch controller. In addition, in order to vary the voltage level, it is required to turn on and turn off one relay, while turning on another relay, so that it needs at least two relays. Moreover, unless the relay operating circuit is connected with a switch (for example a door switch) having direct contacts therebetween in series, when the door of the microwave oven is opened, a heating operation of the microwave oven can be erroneously performed.